1. Field of the Invention
The present invention relates to a taking lens system, and more particularly to a compact taking lens system suitable for use in a digital input apparatus (such as a digital still camera or digital video camera) that takes in an image of a subject by means of a solid-state image sensor.
2. Description of the Prior Art
In recent years, as personal computers and the like become more and more popular, digital still cameras and digital video cameras (hereinafter collectively referred to as “digital cameras”), which permit easy entry of image data into digital equipment, have been becoming more and more popular for personal use among general users. Such digital cameras are expected to become even more widespread as image data input devices in the future.
Solid-state image sensors, such as CCDs (charge-coupled devices), used in digital cameras have been made increasingly small, and accordingly further miniaturization is sought in digital cameras themselves. As a result, miniaturization is eagerly sought also in taking lens systems, which occupy the largest volumes in digital input devices. The easiest way to make a taking lens system smaller is to make a solid-state image sensor smaller. However, this involves making photosensitive elements smaller, and thus makes the solid-state image sensor more difficult to fabricate, and in addition requires higher performance in the taking lens system.
On the other hand, making a taking lens system smaller while keeping the size of a solid-state image sensor unchanged inevitably brings the exit pupil position closer to the image plane. When the exit pupil position is brought closer to the image plane, the off-axial rays exiting from the taking lens system are obliquely incident on the image plane. This makes it impossible to make the most of the light-condensing ability of the microlenses provided in front of the solid-state image sensor, with the result that the obtained image shows extremely uneven brightness between in a central portion and in a peripheral portion thereof This can be avoided by bringing the exit pupil position of the taking lens system farther away from the image plane, but this inevitably makes the taking lens system as a whole unduly large.
Furthermore, in keen competition for lower prices in recent years, lower costs have been increasingly eagerly sought in taking lens systems. To meet these requirements, U.S. Pat. No. 6,414,802 and Japanese Patent Applications Laid-Open No. 2002-98889 propose taking lens systems composed of three lens elements for use with solid-state image sensors.
However, the taking lens system proposed in U.S. Pat. No. 6,414,802 is composed solely of plastic lens elements, and thus has the disadvantage of a large deterioration in performance against variation in temperature. Moreover, this taking lens system has an angle of view of about 50 degrees, which is insufficient for it to be used as a taking lens system. On the other hand, the taking lens system proposed in Japanese Patent Application Laid-Open No. 2002-98889 is composed of two glass lens elements and one plastic lens element, but this construction, despite using two glass lens elements, has the disadvantage of a large deterioration in performance against variation in temperature. Moreover, the use of two glass lens elements results in high costs.